Runners have long feared lactic acid. We've always viewed it as something that tires us out during a race and makes us sore afterward. In high school, I can remember my buddy Jeff propping his legs up after a race and shaking them back and forth, explaining, "I'm draining the lactic acid out of my legs." In college I would endure long massages, believing that it would flush the evil brew out of my muscles, taking my soreness with it. It was like a bad houseguest that would hang around causing trouble until it was physically removed.

In recent years, studies have shown that we've had it all wrong. Most lactic acid is quickly removed after exercise, and it isn't to blame for postrace soreness. (That's the result of microtrauma to the muscles.) It isn't useless either; it contains an important fuel source for high-intensity running. With the right training, we can delay the onset of lactic acid accumulation, and improve our body's capacity to use it for fuel.

Lactic acid is a byproduct that's created when we burn glycogen without oxygen as we run. The higher the intensity of the run, the more lactic acid we create. In the blood, it breaks down into lactate and hydrogen ions. The lactate gets processed and converted into fuel by the mitochondria, energy factories in our cells. No problem there. It's those hydrogen ions that cause all the trouble.

When hydrogen ions accumulate, it becomes difficult for the muscles to contract. That makes running feel more strenuous, and causes that distinct burning sensation we feel after a challenging workout. Fortunately, we can train our bodies to become better at buffering hydrogen ions so we can run harder and delay the time it takes for that burning to set in. We do that by running beyond our lactate threshold-the intensity at which lactic acid begins to accumulate in our blood. This also trains the body to become better at using lactic acid as a fuel.

In a 2006 study at the University of Western Australia, researchers found that athletes who worked out at 120 to 140 percent of lactate threshold three days a week for five weeks improved their ability to buffer hydrogen ions by about 25 percent. In athletes who worked at a lower intensity-95 percent of lactate threshold-buffer capacity stayed the same.

By running beyond your lactic-acid threshold with the workouts below, you train your body to more efficiently process it into a fuel, and at the same time, improve your capacity to buffer those hydrogen ions. All of which will help you run faster, longer.

Fast Work, Big Rewards

Try one of these workouts every two or three weeks instead of an interval session. Run the repeats at your 800-meter pace, or at about 90 percent of all-out effort.